This invention relates generally to in-line electron guns for color cathode ray tubes and specifically to a uni-bipotential in-line electron gun having minimal comatic aberrations.
In-line electron guns for cathode ray tubes are well known in the art. U.S. Pat. No. 5,170,101, assigned to Zenith Electronics Corporations describes an in-line electron gun for use in high resolution tri-beam color cathode ray tubes that have self convergent yokes. As is well known, such yokes undesirably introduce beam shape distortions which, however, may be compensated by various electron lens configurations, one such a being a dynamic lens, often referred to as a Dynamic Quadropole. Other types of distortion are also introduced to the electron beams by the electronic lenses of the gun. A major problem is that of spherical aberration, which is characterized by an increase in focussing power that is proportional to the cube of the radial distance from the optical center of the lens. Thus the outer portion of the beam will be more strongly focussed than the mid portion, and the mid portion more strongly focussed than the center portion, resulting in an aberrated image. To compensate for this distortion, the lenses are made as large as possible, constrained by the electron gun and tube geometry.
Another type of distortion, referred to as comatic distortion, is addressed by the present inventions. Comatic distortion results when the beam is off center of the optical lens axis, that is, the optical axis defined by the electric field of the lens. Comatic distortion, a second order effect, describes a result in which the asymmetric focussing action of the lens causes a tail to appear in the electron spot, compromising the resolution of the cathode ray tube.
This type of aberration is inherent in electron main lens systems referred to as the common or open type where a single large aperture provides part of the focussing action for all three beams passing within. It is readily apparent from symmetry principles that the center beam axis is coincident with the electrical axis of such a main lens and no comatic aberration is generated. However the outer beams, while coincident with the electrical symmetry axis of the main lens in the vertical direction, are generally not coincident with the main lens electrical axis in the horizontal direction and generate a horizontally directed comatic aberration in the outer beams, if not compensated.
In one prior art design using the common lens system, comatic aberration is compensated by placing field corrector electrodes behind the single large apertures forming the main lens. The electrode shapes the electric field such that the electric axis is moved to coincide with that of the beam. This type of corrector electrode has a complex aperture shape that must be very accurately positioned in the gun, making manufacture and assembly difficult.
Another prior art design uses a simple plate with three circular apertures to provide some degree of comatic correction by reducing the diameters so that more focussing action is done by the circular apertures and less by the large single common lens aperture. However, in order to achieve a reasonable amount of correction for comatic aberration, the circular diameter must be decreased to the point where spherical aberration increases. Thus this type of design results in a compromise between comatic aberration and spherical aberration.
In the present invention the diameters of the previously described circular apertures of the corrector electrode are maximized so that their size is restricted only by the physical requirement of having three complete circular apertures. Thus the problem of increased spherical aberration is eliminated. This however causes the electric axis of the main lens to be non-coincident with beam axis and results in comatic aberration. In the present invention this comatic aberration is eliminated by deflecting the beam upstream of the main lens area by offsetting the center of one or more outer beam apertures in the pre-focus region of the gun. The amount and direction of the deflection is chosen so that the outer beams will arrive at the main lens area aligned with the electrical axis of the main lens and thus incur little comatic aberration.
Therefore the present invention retains the manufacturability of the circular aperture corrector electrode while providing a high degree of comatic correction and a low degree of spherical aberration.